Mum's/Lizzie (spoilers)
Sorry guys, if someone brought this up already, but I am very curious and definely I will have to read the book now, what an amazing film.
In one of the last scenes, while Conor and his grandmom are saying goodbye to Lizzie and the MOnster shows in the background, you can see she can see him and she looks very peaceful about it, furthermore, while Conor is exploring his new room at his grandma's house, you can see that the pics that were used to ilustrate the Monster's tales were actualy drwan by his mom in her sketch book.
So she saw it as well as a child?? I am assuming that the Monster also helped her dealing with her dad's passing?